The present invention relates to an analog to digital converter for processing a video signal and, more specifically, to a converter for use in a television system in which a gamma correction is either removed from a gamma corrected video signal or incorporated into a video signal.
Gamma correction is the nonlinear amplitude transformation carried out on a video signal primarily for color television transmission. The primary function of gamma correction is to control the overall amplitude transfer characteristic of the television system between the camera scene and the reproduced picture. Cathode ray picture tubes have transfer characteristics of approximately the form Y=k E.sup.2.2 over most of their useful range of operation, E being the gun drive voltage above cutoff and Y the displayed luminance. The provision of a transducer at the video source having a transfer characteristic of the form E.sub.o =k Y.sub.i.sup..gamma., E.sub.o being the amplitude of the output video signal and Y.sub.i being the original scene luminance respectively, and .gamma. being the gamma correction and being equal to 1/2.2, renders the television system essentially linear if it contains no nonlinear transducers other than the gamma corrector and the picture tube. Accordingly, as is well known, the standard NTSC video signal, which is E.sub.o in the above equation, is proportional to the original scene luminance raised to the 1/2.2 power.
In a television system which does not use a cathode ray tube for a display device and which uses an NTSC signal, it becomes necessary to remove the gamma correction from the NTSC video signal for optimum picture reproduction. One method of effecting this removal is the provision of a nonlinear analog signal transducer in series or parallel with the NTSC video signal source. While this method is generally satisfactory, it has several limitations, including limited dynamic range, the less than optimum accuracy of gamma correction removal over the dynamic range of the transducer, and limited bandwidth. Also, in some television receiver systems, such as most plasma panel display systems, it is also necessary to convert the video signal into digital form. One known method for removing the gamma correction in such a receiver is to linearly convert the analog video signal into digital form and then to use a digital processing scheme to raise the digital value to the 1/.gamma. power. While this method provides the desired result, it has the disadvantages of requiring relatively complex circuitry and being expensive in providing sufficient speed for video data rates.
Also, in television systems which do use a cathode ray picture tube and which do not use the standard NTSC signal it becomes necessary to introduce a gamma correction by raising the video signal to the .gamma. power. Also, the video signal is frequently converted into digital form, such as for transmission or storage. In such instances, the previously mentioned methods for removing the gamma correction may be used for providing the actual gamma correction, but they also have essentially the same limitations as previously discussed.